Charged particle selector

ABSTRACT

The electrostatic selector of aerosol particles of an atmosphere has a first (18) and a second (20) spaced, parallel, coaxial conductive disks between which is established an electrical field, an annular slot (22) made in the first disk in order to communicate with the atmosphere to be examined, a central intake (26) for bringing about a circulation from the periphery of the disks of a stable, centripetal, laminar filtered air flow, the second disk (20) having a central extraction orifice (28).

TECHNICAL FIELD

The present invention relates to a charged particle electrical mobilityselector more particularly used in the case of fine particles forselecting particles having a given grain size (monodisperse aerosol)from particles of a random grain size (polydisperse aerosol), which aresuspended in air or another gas.

Among other applications this type of apparatus is e.g. particularlysuitable for use in the field of aerosol research, particularly forproducing calibrated particles or for studying the electrical charge ofthe aerosols. It is more especially suitable for submicron aerosols downto the smallest aerosol sizes, i.e. approximately 1 nanometer (10⁻⁹ m).

PRIOR ART

One of the most frequently used means for selecting monodisperseparticles is based on the fact that aerosols are carriers of electricalcharges equal to the unitary electrical charge or a multiple thereof.

Consequently for the selection of charged particles suspended in a gasuse has already been made of electrostatic fields acting on theelectrical charges which they carry. In this connection there iscurrently a definition of a fundamental notion in this field, which isthat of the electrical mobility of a charged particle placed in anelectrostatic field. This quantity, which defines the largest orsmallest aptitude of such a particle to undergo a deviation under theeffect of said field can be represented by the following equation:

    W=ZE

in which W is the drift velocity acquired by the particle under theinfluence of the electrical field E to which it is exposed. Theproportionality coefficient Z between the two aforementioned quantitiesis the electrical mobility in question. This electrical mobility is onthe one hand proportional to the electrical charge of the particle andon the other inversely proportional to its diameter, so that it ispossible to produce true selectors of particles as a function of theirelectrical mobility consisting of subjecting particles entrained in agaseous flow to the action of an electrical field between twoelectrodes. Under the effect of the field, the charged particles aredeposited, as a function of their sign, on one of these electrodes andthe abscissa of their deposition with respect to the direction of thegaseous flow is characteristic of their mobility in the sense that thehigher said mobility, the more the abscissa of their deposition is closeto the origin of the gaseous flow carrying the same. This leads to aspread or separation in space of the collected particles. Following saidseparation, it is possible to select the particles having a certainmobility, i.e. a certain grain size if the charge law is known.

An apparatus based on this principle is described in French certificateof addition 90 02413 of 27.2.1990, published under No. 2 658 916 (U.S.Pat. No. 5,117,190) and entitled "Electrostatic aerosol particle sensorand equipments incorporating this application". This type of apparatusis illustrated in FIG. 1 and has two spaced, parallel, coaxialconductive disks 2, 4 between which is established a potentialdifference V and therefore an electrical field E. The disk 2 has anannular slot 6 (radius r₁) by means of which are introduced theparticles of an aerosol at a rate q₁. A central intake 8 is provided bymeans of which an air flow Q circulates under the effect of a not shownpump.

The particles are entrained to a second annular slot 10 of radius r₂formed in the disk 4 under the combined action of a filtered air flow atthe rate q₀, which is radial and laminar and established between the twodisks, as well as the electrical field E imposed between the two disks.

Through the slot 10, the air flows at a rate q₂ into a cylindrical box12 fixed beneath the disk 4 giving Q=q₀ +q₁ -q₂. The particlestraversing the slot 10 have the same electrical mobility Z=Q /πE(r₁ ²-r₂ ²). For information, the order of magnitude of the flow rates is q₁=q₂ =Q/10.

In order to regulate this electrical mobility to the desired value foreach individual case, it is possible to act on two parameters, namelythe flow rate Q on the one hand and the potential difference V appliedbetween the two coaxial conductive disks 2 and 4 on the other.

By means of the cylindrical box 12 and a pipe 14, the particles can thenbe directed towards any type of device appropriate for the envisagedapplication, e.g. a particle counter to be calibrated with the thusproduced, fine, calibrated particles.

This type of apparatus, like all known charged particle selectors,suffers from certain disadvantages if it is used for selecting particleshaving a nanometric size.

Firstly, the transport of such particles in circuits of the system leadsto losses as a result of Brown scatter in the vicinity of the walls,particularly in the cylindrical box 12.

Moreover, the transit time in this box, which can be considerable, isnot the same for all the particles and consequently there is a certainscatter through the apparatus, which can be prejudicial in certainapplications.

DESCRIPTION OF THE INVENTION

The invention aims at solving these problems.

It relates to a selector of aerosol particles contained in an atmospherecomprising a first and a second spaced, parallel, coaxial conductivedisks between which it is possible to establish an electrical field byraising them to different potentials, the space between the two diskscommunicating with the atmosphere to be examined through an annular slotof radius r₁ made in the first disk, a central intake being provided inthe first disk for bringing about the circulation in said space, fromthe periphery of the disk, of a stable, centripetal, laminar filteredair flow, characterized in that the second disk has a central extractionorifice.

By means of said central orifice, it is possible to select aerosolparticles having a clearly defined electrical mobility.

In addition, compared with known apparatuses, said central extractiondevice is easy to produce, whilst guaranteeing better performancecharacteristics, with in particular less fine particle losses and ashorter transit time in the extraction circuit.

Other embodiments of the invention can be gathered from the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention can be better gathered fromthe following description of non-limitative embodiments and withreference to the attached drawings, wherein show:

FIG. 1 A prior art aerosol particle selector.

FIGS. 2 and 3 The principle of a particle selector according to theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 2 is a diagram of a selector according to the invention. Theapparatus comprises two concentric disk 18, 20. The first disk 18 has anannular slot 22 of radius r₁ by means of which a gas sample containingthe particles to be selected is introduced with a flow rate q₁. As thetwo disks are conductive, each of them can be raised to a certainpotential and an electrical field E is established in the space 24separating them. In the second disk 20 is made a central, circularorifice 28 to be traversed by an air flow q₂. Between the two disks,onto the periphery of the space 24 and using means not shown in FIG. 2,is injected an entrainment gas (filtered air), q_(o), so that a laminarflow circulates between the two disks, up to the central suction pipe26, which consequently absorbs a flow Q=q₀ +q₁ -q₂. The selectedparticles can then be discharged by a pipe 30 to the outside of theselector.

The central extraction orifice 28 represents a vital point both from theaeraulic standpoint and from the electrical standpoint, as illustratedin FIG. 3.

If it is firstly assumed that there is no extraction orifice 28, it isthen possible to define a point A at the intersection of the centralaxis Δ of the apparatus and the disk 20. This point A is a vitalaeraulic point because it corresponds to the stoppage point of thelaminar flow. It is also a vital point from the electrical standpoint,because the field lines are poorly defined at this location, whereasthey are perfectly defined between the two disks elsewhere (except ontheir periphery, but this has no influence because there are no aerosolparticles).

Therefore the basis for the provision of an extraction orifice 28 atpoint A is not particularly obvious. However, it has been found thatthis configuration makes it possible to select aerosol particles with aclearly defined electrical mobility given by Z=Q/πEr₁ ².

The diameter .o slashed. of the orifice 28 is preferably chosen so as toensure a good selectivity of the particles (if the diameter is large theselectivity is low), but also so as not to disturb the flow of fluidtraversing it (if the diameter is small, there can be an excessivepressure drop at said orifice).

The particles extracted from the selector can then be counted by adetector chosen from among known systems usually based on optical orelectrical methods.

The described apparatus has as its essential application the productionof monodisperse particles from polydisperse sources in the submicronparticle range. In order to produce particles of a given size, it ismerely necessary to select them as a function of the mobility Zcorresponding to said size and therefore choose the appropriate field Eand flow rate Q.

I claim:
 1. Electrical mobility selector of charged aerosol particlescontained in an atmosphere to be examined comprising a first and asecond, spaced, parallel, coaxial conductive disks; means forestablishing an electrical field E between said disks by raising thedisks to different potentials; an annular slot of radius r₁ in the firstdisk, the space between the two disks communicating with the atmosphereto be examined through the annular slot; a central intake provided inthe first disk for bringing about the circulation in said space, fromthe periphery of the disk, of a stable, centripetal, laminar filteredair flow; and a central extraction orifice in the second disk. 2.Selector according to claim 1, further comprising a pipe fixed to thesecond disk, the particles being discharged to the outside of theselector by said pipe.
 3. Selector according to one of the claims 1 or2, wherein the diameter of the central extraction orifice is chosen soas to ensure the selectivity of the particles.
 4. Electrical mobilityselector of charged aerosol particles contained in an atmosphere to beexamined comprising a first and a second, spaced, parallel, coaxialconductive disks; means for establishing an electrical field E betweensaid disks by raising the disks to different potentials; an annular slotof radius r₁ in the first disk, the space between the two diskscommunicating with the atmosphere to be examined through the annularslot; a central intake provided in the first disk for bringing about thecirculation in said space, from the periphery of the disk, of a stable,centripetal, laminar filtered air flow; and a central extraction orificein the second disk having a diameter so as to ensure selectivity of theparticles.